Nanowires or nanopyramids grown on graphitic substrate

ABSTRACT

A composition of matter comprising: a graphitic substrate optionally carried on a support, a seed layer having a thickness of no more than 50 nm deposited directly on top of said substrate, opposite any support; and an oxide or nitride masking layer e directly on top of said seed layer; wherein a plurality of holes are present through said seed layer and through said masking layer to C said graphitic substrate; and wherein a plurality of nanowires or nanopyramids are grown from said substrate in said holes, said nanowres or nanopyramids comprising at least one semiconducting group III-V compound.

This invention concerns the fabrication of a hole patterned mask layeron a thin graphitic layer as a transparent, conductive and flexiblesubstrate for nanowire or nanopyramid arrays preferably grown by a metalorganic vapour phase epitaxy (MOVPE) method or molecular beam epitaxy(MBE) method. The graphitic substrate is provided with a seed layerwhich can be patterned to allow nanowire or nanopyramid growth in apatterned form such as a nanowire or nanopyramid array. Alternatively,the seed layer is itself provided with a masking layer which can bepatterned (along with the seed layer) to allow nanowire or nanopyramidgrowth. The graphitic layer with seed and optionally masking layer ontop can be transferred from a substrate onto other support surfaces,which may enhance vertical nanowire or nanopyramid growth.

Over recent years, the interest in semiconductor nanowires hasintensified as nanotechnology becomes an important engineeringdiscipline. Nanowires, which are also referred to as nanowhiskers,nanorods, nanopillars, nanocolumns, etc. by some authors, have foundimportant applications in a variety of electrical devices such assensors, solar cells to LED's.

For the purpose of this application, the term nanowire is to beinterpreted as a structure being essentially in one-dimensional form,i.e. is of nanometer dimensions in its width or diameter and its lengthtypically in the range of a few 100 nm to a few μm. Usually, nanowiresare considered to have at least two dimensions not greater than 500 nm,such as not greater than 350 nm, especially not greater than 300 nm suchas not greater than 200 nm.

Many different types of nanowires exist, including metallic (e.g., Ni,Pt, Au, Ag), semiconducting (e.g., Si, InP, GaN, GaAs, ZnO), andinsulating (e.g., SiO₂, TiO₂) nanowires. The present inventors areprimarily concerned with semiconductor nanowires although it isenvisaged that the principles outlined in detail below are applicable toall manner of nanowire technology.

Conventionally, semiconductor nanowires have been grown on a substrateidentical to the nanowire itself (homoepitaxial growth). Thus, GaAsnanowires aregrown on GaAs substrates and so on. This, of course,ensures that there is a lattice match between the crystal structure ofthe substrate and the crystal structure of the growing nanowire. Bothsubstrate and nanowire can have identical crystal structures. Thepresent invention, however, concerns nanowires grown on graphiticsubstrates (heteroepitaxial growth).

Graphitic substrates are substrates composed of single or multiplelayers of graphene or its derivatives. In its finest form, graphene is aone atomic layer thick sheet of carbon atoms bound together with doubleelectron bonds (called a sp² bond) arranged in a honeycomb latticepattern. Graphitic substrates are thin, light, and flexible, yet verystrong.

Compared to other existing transparent conductors such as ITO,ZnO/Ag/ZnO, TiO₂/Ag/TiO₂, graphene has been proven to have superioropto-electrical properties as shown in a recent review article in NaturePhotonics 4 (2010) 611.

The growth of nanowires (NWs) on graphene is not new. In WO2012/080252,there is a discussion of the growth of semiconducting nanowires ongraphene substrates using MBE. WO2013/104723 concerns improvements onthe '252 disclosure in which a graphene top contact is employed on NWsgrown on graphene.

For many applications it will be important that the nanowires ornanopyramids can be grown vertically, perpendicular to the substratesurface.

Semiconductor nanowires normally grow in the [111] direction (if cubiccrystal structure) or the [0001] direction (if hexagonal crystalstructure). This means that the substrate surface needs to be (111) or(0001) oriented where the surface atoms of the substrate is arranged ina hexagonal symmetry.

One problem, however, is that nanowires or nanopyramids can growrandomly on a substrate, in any position or in any direction. In orderto position the nanowires therefore, it is known to use a mask with ahole array pattern where nanowires are allowed to grow only in thehole-patterned area. The mask can also promote NW growth in a directionperpendicular to the substrate. Typically, a silica layer is applied toa substrate and etched to create holes in a desired pattern. Nanowiresthen grow only at the location of the holes. In Nano Letters 14 (2014)960-966, Munshi et al. show GaAs nanowires grown on Si using a silicamask. Other publications such as Journal of Crystal Growth 310 (2008)1049-56 also describe masked crystal growth. In Nanotechnology 22 (2011)275602, Plissard et al. describe a nanowire positioning technique basedon gallium droplet positioning. Nanowires only grow from the Ga dropletsso their positions can be controlled.

The present inventors have realised, however, that the deposition of aconventional silica or silicon nitride mask on a graphitic substrate isproblematic. Masks can be made of an inert compound, such as oxides andnitrides. In particular, the hole-patterned mask comprises at least oneinsulating material such as SiO₂, Si₃N₄, HfO₂, or Al₂O₃ e.g. Thesematerials can be deposited on general semiconductor substrates bychemical vapour deposition (CVD), plasma-enhanced CVD (PE-CVD),sputtering, and atomic layer deposition (ALD) in high quality. However,these deposition methods are not readily applicable to graphiticsubstrates. Deposition of oxides or nitrides materials by CVD alwaysinvolves the highly reactive oxygen or nitrogen radicals, which caneasily damage the carbon bonds in graphene, leading to a serious loss ofuseful properties such as high electrical conduction. The damage becomesmore severe if the radicals are further activated in a plasma form inPE-CVD deposition. This problem is quite similar in sputtering wherehighly accelerated oxides/nitrides elements in plasma bombards thegraphitic surface.

In addition, graphitic substrates are free of dangling bonds on thesurface, therefore chemically inert with a hydrophobic nature. Thismakes it difficult to deposit oxides and nitrides with e.g. H₂O-basedALD technique, requiring complicated chemical functionalization of thegraphene surface, which again degrades its properties.

The present inventors therefore propose the application of a seed layeron the graphitic substrate before the application of a masking layer orbefore conversion of the seed layer into an oxide layer and patterningfor the positioned growth of NWs or nanopyramids.

The inventors have appreciated that a thin seed layer can be depositedon a graphitic substrate without damaging the surface of that substrate.The thin seed layer also protects the substrate from the deposition ofother unwanted materials thereon.

In particular, the seed layer is an inert layer which does not reactwith graphene. This seed layer can then be oxidised to form an oxidelayer or the seed layer can form a support for the deposition of amasking layer. These layers can then be etched to form holes for NW ornanopyramid growth. The overall solution is therefore much lessaggressive and leads to fewer defects on the graphene and less risk ofextraneous contamination.

The application of a thin seed layer on graphene for high quality oxidedeposition is not new. In Appl. Phys. Lett. 94 (2009) 062107, Kim et al.describe the deposition of a thin aluminium layer before depositingaluminium oxide on graphene by ALD. No one before, however, hasconsidered the importance of the seed layer in the context of controlledgrowth of nanowires or nanopyramids on graphitic substrates.

In addition, the inventors have found that the application of seed andmasking layers can be used as a “carbon contamination-free” scaffold forgraphene when the transfer of graphene is desired for NW or nanopyramidgrowth.

Growth of single and multi-layered graphene has been successfullydemonstrated on different substrates. Using the Si sublimation at hightemperature, graphene can be grown on SiC substrates. The CVD growth ofgraphene is the well-known technique, where the most popular way is tomake use of metal catalysts such as Cu, Ni, Pt, Ru, etc. Recently thegrowth of graphene on semiconductors such as Ge and Si as well as oninsulators such as SiO₂ and Al₂O₃ has been reported.

The inventors have noted that as-grown graphene with the metal catalystunderneath may not be readily used for NW growth. For example, the useof CVD graphene grown on Cu cannot be used for NW growth as the Cu cancause severe contamination in the grown NWs, caused inter alia in thegrowth chamber from the evaporation and out-diffusion of Cu at hightemperature. CVD grown graphene on metal catalysts usually has localdefects and micro- (or nano) cracks where the bottom metal surface isexposed. The exposed metal surface can be highly reactive withsemiconductor materials during the NW growth, which can overwhelm anddestroy the proper NW growth on graphene surface. Therefore the transferof CVD grown graphene from the metal catalyst surface to other surfacesmay be necessary before NW growth.

Using the seed (and possible mask) layers as the scaffold for graphenetransfer has another important merit. The layers can be depositeddirectly after the CVD growth of graphene, and therefore protect theclean graphene surface from further processing that involves thedeposition of polymer-based materials such as e-beam resists forgraphene transfer. The direct deposition of polymer-based materials onCVD grown graphene always leaves carbon residues on the surface, whichresults in carbon contamination during NW growth, degrading the dopingcontrol and optical properties of NWs as well as contaminating thegrowth system. It could also affect the NW growth itself.

So the deposition of seed (and possible mask) layers before depositingpolymer-based materials makes it possible to have the graphene surfacecarbon-contamination free. It can also be beneficial to incorporate thepolymer-based material into the hole patterning process. It can bee-beam resist (or nano-imprinting resist) if the hole patterning wouldbe done by e-beam lithography (nano-imprinting). Any carbon residues onthe seed or mask layers can be thoroughly cleaned by oxygen plasmatreatment and wet chemical etching, which would easily destroy the baregraphene without seed or mask layers.

SUMMARY OF INVENTION

Thus, viewed from one aspect the invention provides a composition ofmatter comprising:

a graphitic substrate optionally carried on a support;

a seed layer having a thickness of no more than 50 nm deposited directlyon top of said substrate, opposite any support; and

an oxide or nitride masking layer directly on top of said seed layer;

wherein a plurality of holes are present through said seed layer andthrough said masking layer to said graphitic substrate; and wherein

a plurality of nanowires or nanopyramids are grown from said substratein said holes, said nanowires or nanopyramids comprising at least onesemiconducting group III-V compound.

Viewed from another aspect the invention provides a composition ofmatter comprising:

a graphitic substrate optionally carried on a support;

an oxidised or nitridized seed layer having a thickness of no more than50 nm present directly on top of said substrate, opposite any support;optionally

an oxide or nitride masking layer directly on top of said oxidised ornitridized seed layer;

wherein a plurality of holes are present through said seed layer andthrough said masking layer, if present, to said graphitic substrate; andwherein

a plurality of nanowires or nanopyramids are grown from said substratein said holes, said nanowires or nanopyramids comprising at least onesemiconducting group III-V compound.

Viewed from another aspect the invention provides a process comprising:

(I) providing a graphitic substrate on a support and depositing thereona seed layer having a thickness of no more than 50 nm;

(II) oxidising or nitridizing said seed layer to form a oxidised ornitridized seed layer; optionally

(III) depositing a masking layer on said oxidised or nitridized seedlayer, e.g. an oxide or nitride masking layer;

(IV) optionally transferring the graphitic substrate to a differentsupport;

(V) introducing a plurality holes in said oxidised or nitridized seedlayer and said masking layer, if present, said holes penetrating to saidsubstrate;

(VI) growing a plurality of semiconducting group III-V nanowires ornanopyramids in the holes, preferably via a molecular beam epitaxy ormetal organic vapour phase epitaxy.

Viewed from another aspect the invention provides a process comprising:

(I) providing a graphitic substrate on a support and depositing thereona seed layer having a thickness of no more than 50 nm;

(II) depositing on said seed layer an oxide or nitride masking layer,

(III) introducing a plurality holes in said seed layer and said maskinglayer said holes penetrating to said substrate;

(IV) optionally transferring the graphitic substrate to a differentsupport;

(V) growing a plurality of semiconducting group III-V nanowires ornanopyramids in the holes, preferably via a molecular beam epitaxy ormetalorganic vapour phase epitaxy.

Viewed from another aspect the invention provides a product obtained bya process as hereinbefore defined.

Optionally, the surface of the graphitic substrate can bechemically/physically modified in the said plurality of holes to enhancethe epitaxial growth of nanowires or nanopyramids.

Viewed from another aspect the invention provides a device, such as anelectronic device, comprising a composition as hereinbefore defined,e.g. a solar cell, light emitting device or photodetector.

Definitions

By a group III-V compound semiconductor is meant one comprising at leastone element from group 111 and at least one element from group V. Theremay be more than one element present from each group. e.g. InGaAs, AlGaN(i.e. a ternary compound), AlInGaN (i.e. a quaternary compound) and soon. The term semiconducting nanowire or nanopyramid is meant nanowire ornanopyramid made of semiconducting materials from group III-V elements.

The term nanowire is used herein to describe a solid, wirelike structureof nanometer dimensions. Nanowires preferably have an even diameterthroughout the majority of the nanowire, e.g. at least 75% of itslength. The term nanowire is intended to cover the use of nanorods,nanopillars, nanocolumns or nanowhiskers some of which may have taperedend structures. The nanowires can be said to be in essentially inone-dimensional form with nanometer dimensions in their width ordiameter and their length typically in the range of a few 100 nm to afew μm. Ideally the nanowire diameter is not greater than 500 nm.Ideally the nanowire diameter is between 50 and 500 nm, however, thediameter can exceed few micrometers (called microwires).

Ideally, the diameter at the base of the nanowire and at the top of thenanowire should remain about the same (e.g. within 20% of each other).

The term nanopyramid refers to a solid pyramidal type structure. Theterm pyramidal is used herein to define a structure with a base whosesides taper to a single point generally above the centre of the base. Itwill be appreciated that the single vertex point may appear chamferred.The nanopyramids may have multiple faces, such as 3 to 8 faces, or 4 to7 faces. Thus, the base of the nanopyramids might be a square,pentagonal, hexagonal, heptagonal, octagonal and so on. The pyramid isformed as the faces taper from the base to a central point (formingtherefore triangular faces). The triangular faces are normallyterminated with (1-101) or (1-102) planes. The triangular side surfaceswith (1-101) facets could either converge to a single point at the tipor could form a new facets ((1-102) planes) before converging at thetip. In some cases, the nanopyramids are truncated with its topterminated with {0001} planes. The base itself may comprise a portion ofeven cross-section before tapering to form a pyramidal structure begins.The thickness of the base may therefore be up to 200 nm, such as 50 nm.

The base of the nanopyramids can be 50 and 500 nm in diameter across itswidest point. The height of the nanopyramids may be 200 nm to a fewmicrometers, such as 400 nm to 1 micrometer in length.

It will be appreciated that the substrate comprises a plurality ofnanowires or nanopyramids. This may be called an array of nanowires ornanopyramids.

Graphitic layers for substrates are films composed of single or multiplelayers of graphene or its derivatives. The term graphene refers to aplanar sheet of sp²-bonded carbon atoms in a honeycomb crystalstructure. Derivatives of graphene are those with surface modification.For example, the hydrogen atoms can be attached to the graphene surfaceto form graphane. Graphene with oxygen atoms attached to the surfacealong with carbon and hydrogen atoms is called as graphene oxide. Thesurface modification can be also possible by chemical doping oroxygen/hydrogen or nitrogen plasma treatment.

The term epitaxy comes from the Greek roots epi, meaning “above”, andtaxis, meaning “in ordered manner”. The atomic arrangement of thenanowire or nanopyramid is based on the crystallographic structure ofthe substrate. It is a term well used in this art. Epitaxial growthmeans herein the growth on the substrate of a nanowire or nanopyramidthat mimics the orientation of the substrate.

Selective area growth (SAG) is the most promising method for growingpositioned nanowires or nanopyramids. This method is different from themetal catalyst assisted vapour-liquid-solid (VLS) method, in which metalcatalyst act as nucleation sites for the growth of nanowires ornanopyramids. Other catalyst-free methods to grow nanowires ornanopyramids are self-assembly, spontaneous MBE growth and so on, wherenanowires or nanopyramids are nucleated in random positions. Thesemethods yield huge fluctuations in the length and diameter of thenanowires and the height and width of nanopyramids. Positioned nanowiresor nanopyramids can also be grown by the catalyst-assisted method.

The SAG method or the catalyst-assisted positioned growth methodtypically requires a mask with nano-hole patterns on the substrate. Thenanowires or nanopyramids nucleate in the holes of the patterned mask onthe substrate. This yields uniform size and pre-defined position of thenanowires or nanopyramids.

The term mask refers to the mask material that is directly deposited onthe seed layer. The mask material should ideally not absorb emittedlight (which could be visible, UV-A, UV-B or UV-C) in the case of an LEDor not absorb the entering light of interest in the case of aphotodetector. The mask should also be electrically non-conductive. Themask could contain one or more than one material, which include Al₂O₃,SiO₂, Si₃N₄, TiO₂, W₂O₃, and so on. Subsequently, the hole patterns inthe mask material can be prepared using electron beam lithography ornanoimprint lithography and dry or wet etching.

Molecular beam epitaxy (MBE) is a method of forming depositions oncrystalline substrates. The MBE process is performed by heating acrystalline substrate in a vacuum so as to energize the substrate'slattice structure. Then, an atomic or molecular mass beam(s) is directedonto the substrate's surface. The term element used above is intended tocover application of atoms, molecules or ions of that element. When thedirected atoms or molecules arrive at the substrate's surface, thedirected atoms or molecules encounter the substrate's energized latticestructure or a catalyst droplet as described in detail below. Over time,the oncoming atoms form a nanowire.

Metalorganic vapour phase epitaxy (MOVPE) also called as metalorganicchemical vapour deposition (MOCVD) is an alternative method to MBE forforming depositions on crystalline substrates. In case of MOVPE, thedeposition material is supplied in the form of metalorganic precursors,which on reaching the high temperature substrate decomposes leavingatoms on the substrate surface. In addition, this method requires acarrier gas (typically H₂ and/or N₂) to transport deposition materials(atoms/molecules) across the substrate surface. These atoms reactingwith other atoms form an epitaxial layer on the substrate surface.Choosing the deposition parameters carefully results in the formation ofa nanowire.

The graphene transfer process is the process generally used to transferas-grown graphene from metal catalyst to other supports. The overallprocess is that, first the polymer-based layer such as e-beam resist andphotoresist is deposited on graphene as a scaffold usually by aspin-coating method with a thickness of 0.1˜1 μm. Then graphene withpolymer layer on top is detached from metal catalyst by either etchingaway the metal catalyst in wet etching solution or electrochemicaldelamination in an electrolyte (Nat. Commun. 3 (2012) 699). The graphenewith a polymer layer on top, which is now floating in the solution, canbe transferred onto desired supports. After the transfer, the polymerlayer can be removed by acetone or further processed as resist fore-beam lithography or nano-imprinting lithography.

DETAILED DESCRIPTION OF INVENTION

This invention concerns the use of graphitic layers as a substrate fornanowire or nanopyramid growth in combination with a seed layer andoptionally a masking layer. Ideally, the graphitic layer is transparent,conductive and flexible.

The semiconductor nanowire or nanopyramid array comprises a plurality ofnanowires or nanopyramids grown epitaxially from said nanowire ornanopyramid substrate.

Having a nanowire or nanopyramid grown epitaxially provides homogeneityto the formed material which may enhance various end properties, e.g.mechanical, optical or electrical properties.

Epitaxial nanowires or nanopyramids may be grown from gaseous, liquid orsolid precursors. Because the substrate acts as a seed crystal, thedeposited nanowire or nanopyramid can take on a lattice structure andorientation similar to those of the substrate. Epitaxy is different fromother thin-film deposition methods which deposit polycrystalline oramorphous films, even on single-crystal substrates.

Substrate for Nanowire or Nanopyramid Growth

The substrate used to grow nanowires or nanopyramids is a graphiticsubstrate, more especially it is graphene. As used herein, the termgraphene refers to a planar sheet of sp²-bonded carbon atoms that aredensely packed in a honeycomb (hexagonal) crystal lattice. Thisgraphitic substrate should preferably be no more than 20 nm inthickness. Ideally, it should contain no more than 10 layers of grapheneor its derivatives, preferably no more than 5 layers (which is called asa few-layered graphene). Especially preferably, it is a one-atom-thickplanar sheet of graphene.

The crystalline or “flake” form of graphite consists of many graphenesheets stacked together (i.e. more than 10 sheets). By graphiticsubstrate therefore, is meant one formed from one or a plurality ofgraphene sheets.

It is preferred if the substrate in general is 20 nm in thickness orless. Graphene sheets stack to form graphite with an interplanar spacingof 0.335 nm. The graphitic substrate preferred comprises only a few suchlayers and may ideally be less than 10 nm in thickness. Even morepreferably, the graphitic substrate may be 5 nm or less in thickness.The area of the substrate in general is not limited. This might be asmuch as 0.5 mm² or more, e.g. up to 5 mm² or more such as up to 10 cm².The area of the substrate is thus only limited by practicalities.

In a highly preferred embodiment, the substrate is single layer ormulti-layer graphene grown on metal catalysts by using a chemical vapourdeposition (CVD) method. Metal catalysts can be metallic films or foilsmade of e.g. Cu, Ni, or Pt. Transfer of the graphene layer grown onthese metal catalysts to another substrate can be affected by techniquesdiscussed in detail below. Alternatively, the substrate is a laminatedgraphite substrate exfoliated from Kish graphite, a single crystal ofgraphite, or is a highly ordered pyrolytic graphite (HOPG).

Whilst it is preferred if the graphitic substrate is used withoutmodification, the surface of the graphitic substrate can be modified.For example, it can be treated with plasma of hydrogen, oxygen,nitrogen, NO₂ or their combinations. Oxidation of the substrate mightenhance nanowire or nanopyramid nucleation. It may also be preferable topretreat the substrate, for example, to ensure purity before nanowire ornanopyramid growth. Treatment with a strong acid such as HF or BOE is anoption. Substrates might be washed with iso-propanol, acetone, orn-methyl-2-pyrrolidone to eliminate surface impurities.

The cleaned graphitic surface can be further modified by doping. Dopantatoms or molecules may act as a seed for growing nanowires ornanopyramids. A solution of FeCl₃, AuCl₃ or GaCl₃ could be used in adoping step.

The graphitic layers, more preferably graphene, are well known for theirsuperior optical, electrical, thermal and mechanical properties. Theyare very thin but very strong, light, flexible, and impermeable. Mostimportantly in the present invention they are highly electrically andthermally conducting, flexible and transparent. Compared to othertransparent conductors such as ITO. ZnO/Ag/ZnO, and TiO₂/Ag/TiO₂ whichare commercially used now, graphene has been proven to be much moretransparent (˜98% transmittance in the spectral range of interest from200 to 2000 nm in wavelength) and conducting (<1000 Ohm/□ sheetresistance for 1 nm thickness).

Support for Substrate

The graphitic substrate may need to be supported in order to allowgrowth of the nanowires or nanopyramids thereon. The substrate can besupported on any kind of material including conventional semiconductorsubstrates and transparent glasses.

Conventional semiconductor substrates can be crystalline Si and GaAswith a crystal orientation of [111], [110], or [100] perpendicular tothe surface. They can also have oxide or nitride layers such as SiO₂,Si₃N₄ on top. Some examples of other support substrates include fusedsilica, fused quartz, silicon carbide, fused alumina or AlN. The supportshould be inert. After nanowire or nanopyramid growth and before use ina device, the support might be removed, e.g. by peeling away the supportfrom the graphitic substrate.

Seed Layer & Masking Layer

The invention requires the application of a thin seed layer on thegraphitic substrate. That seed layer may be metallic, semiconducting orinsulating. That seed layer is preferably deposited using thermal ore-beam evaporation. Sputtering, CVD or PE-CVD may be possible as long asit does not degrade the graphene surface. For example, a remote plasmatechnique where the graphene surface does not expose to the directplasma of seed material having high kinetic energy, but only low-energy,diffused seed material can selectively be deposited with less damage.

The seed layer should be no more than 50 nm in thickness, such as nomore than 40 nm, especially no more than 30 nm. The seed layer can intheory be as thin as possible to protect the substrate from damage, suchas 1 or 2 nm. It may have a minimum thickness of 1 nm. An especiallypreferred option is 2 to 20 nm in thickness, which can be easily checkedby scanning electron microscopy after deposition.

Semiconducting seed layers of interest are those based on group III-Velements, such as those described below in connection with the nanowiresor nanopyramids being grown, as well as group IV elements such as Si andGe. It is however preferred if the seed layer is formed from a singleelement. Ideally, that element is a metallic element which term shallinclude Si in this instance. The metal used to form the metallic seedlayer is preferably a transition metal, Al, Si, Ge, Sb, Ta, W, or Nb, Bmay also be used. Ideally a first row transition metal (e.g. first rowtransition metal), Si or Al is used. Ideally, it is Al, Si, Cr or Ti. Itwill be appreciated that there should be no reaction between the seedlayer material and the substrate. There is a risk that Al may oxidisethe graphene so Al is preferably avoided.

The seed layer may be formed from a plurality of layers if desired,perhaps to ensure ideal adhesion between the graphitic substrate whichlies underneath the seed layer and the masking layer which is depositedon top. It may be that the same seed layer material is not ideal foradhering to both these other layers and hence a stack of seed layersmight be used.

So whilst a multiple of seed layers could be used, these are stillpreferably based on a metallic element. Also, it is essential that thetotal thickness of the seed layer is 50 nm or less.

Once the seed layer has been deposited, there are two options. The seedlayer itself can be oxidised or nitridized or a masking layer can bedeposited on top of the seed layer.

In the first embodiment, the seed layer is exposed to oxygen or nitrogento cause oxidation or or nitridization of the seed layer to thecorresponding oxide or nitride. The oxygen/nitrogen can be supplied aspure gas but more conveniently, it is simply supplied in air. Thetemperature and pressure of the oxidation process can be controlled toensure that the seed layer oxidises/nitridizes but not the graphiticlayer. Oxygen/nitrogen plasma treatment can be also applied. Preferredoxides are silicon dioxide, titanium dioxide or aluminium oxide.

In the second embodiment an oxide or nitride masking layer, preferably ametal oxide or metal nitride layer or semimetal oxide or semimetalnitride) is deposited on top of the seed layer. This can be achievedthrough atomic layer deposition or the techniques discussed above inconnection with the deposition of the seed layer. The oxide used ispreferably based on a metal or semimetal (such as Si). The nature of thecation used in the masking layer may be selected from the same optionsas the seed layer i.e. Al, Si or a transition metal, especially a first3d row transition metal (Sc—Zn). The masking layer can therefore beformed from an oxide or a nitride of the seed layer element It ispreferred if the metal atom of the seed layer (adjacent the maskinglayer) is the same as the cation of the masking layer. The masking layershould, however, be formed of a different material to the seed layer.

Preferred masking layers are based on oxides, such as SiO₂, Si₃N₄, TiO₂or Al₂O₃, W₂O₃, and so on.

It is within the scope of the invention for a second masking layer to beapplied on top of the first masking layer, especially when Al₂O₃ isemployed as a lower masking layer. Again, the materials used in thislayer are oxides or nitrides such as metal oxides or nitrides oftransition metals, Al or Si. The use of silica is preferred. It ispreferred if the second masking layer is different from the firstmasking layer. The use of atomic layer deposition is appropriate toapply that second masking layer or the same techniques described withthe seed layer deposition can be employed.

Each of the masking layers may be 5 to 100 nm in thickness, such as 10to 50 nm. There may be a plurality of such layers, such as 2, 3 or 4masking layers.

The total thickness of seed layer and masking layers may be up to 200 nmsuch as 30 to 100 nm.

The seed layer and masking layers are preferably continuous and coverthe substrate as a whole. This ensures that the layers are defect-freeand thus prevents nucleation of nanowires or nanopyramids on theseed/masking layer.

In a further embodiment, a masking layer as hereinbefore defined can beapplied to an oxidised or nitridized seed layer as hereinbefore defined.For example a silicon dioxide layer might be applied by PE-CVD onto anoxidised silicon seed layer. Again, the masking layer might be 5 to 100nm in thickness such as 10 to 50 nm.

The seed layer or masking layer should be smooth and free of defects sothat nanowires or nanopyramids cannot nucleate on the seed layer.

Transfer of Graphene with Seed (and/or Masking) Layer

The CVD growth of single and multi-layer graphene using metal catalystsupports such as Cu, Pt, and Ni in foil or film form, is a quitewell-matured process. In order to use graphene in device fabrication, itis preferred to transfer graphene by detaching it from the metalcatalyst to another support such as one hereinbefore defined. The mostcommon way to do this is to transfer the graphene using a wet etchingmethod where, for example, CVD grown graphene on Cu foil is the basewhere e-beam resist is first deposited as a scaffold and then immersedin a Cu etchant solution. Then the CVD graphene/e-beam resist layersremain floating in the etching solution and can be transferred to othersubstrates. However, this method always leaves significant contaminationon the transferred graphene from residual Cu which comes from incompletewet etching of the Cu foil or re-deposition of Cu on graphene during theCVD growth.

Additional contamination would be present as carbon remnants from thee-beam resist scaffold. These can be detrimental in NW or nanopyramidgrowth, contaminating the NWs or nanopyramids as well as the growthsystem. With the deposition of a seed layer (or seed and mask layers) onCVD grown graphene before depositing the polymer-based layer scaffoldsuch as e-beam resist, the contamination issue by carbon remnants on thegraphene surface can be removed. It is preferable to use the CVD growngraphene on Pt with an electrochemical delamination method for graphenetransfer. Pt has a very high melting temperature (T>1500° C.) with avery low vapour pressure (<10⁻⁷ mmHg) at the graphene growth temperatureof ˜1000° C. The electrochemical delamination method is a method wheregraphene is delaminated from the metal catalyst surface by hydrogenbubbles generated at the cathode, which here would be the graphene/Ptstack, by applying a voltage in an electrolyte solution. The anode wouldbe made of Pt as well. It does not involve any etching of Pt. Thereforethis would not give any Pt remnants on the grown graphene, whichconsequently does not raise any contamination issue in NW or nanopyramidgrowth.

There is also a possibility to make use of the polymer scaffold forsubsequent patterning processes. If the polymer scaffold used is ane-beam resist, it can be directly used for the e-beam lithography ofhole patterning after the transfer to a support without any otherprocesses except a drying step.

Thus viewed from another aspect the invention provides a processcomprising:

(I) providing a graphitic substrate composed of CVD grown single ormulti-layer graphene on a metal catalyst layer, such as Pt, anddepositing thereon a seed layer having a thickness of no more than 50nm;

(II) oxidising or nitridizing said seed layer to form an oxidised ornitridized seed layer, optionally

(III) depositing a masking layer on said oxidised or nitridized seedlayer;

(IV) depositing a polymer layer on said masking layer (if present) orsaid oxidised seed or nitridized layer, said polymer layer being capableof acting as a scaffold for the transfer of said graphitic substrate toanother support;

(V) transferring the graphitic substrate from said metal catalyst layerto another support;

(VI) optionally removing the polymer layer, and optionally depositing afurther oxide or nitride masking layer on top of the upper layerpresent:

(VII) introducing a plurality holes through all layers presentpenetrating to said graphitic substrate;

(VIII) growing a plurality of semiconducting group III-V nanowires ornanopyramids in the holes, preferably via a molecular beam epitaxy ormetalorganic vapour phase epitaxy.

Viewed from another aspect the invention provides a process comprising:

(I) providing a graphitic substrate composed of CVD grown single ormulti-layered graphene on a metal catalyst layer, such as Pt, anddepositing thereon a seed layer having a thickness of no more than 50nm;

(II) depositing a masking layer on said seed layer;

(III) depositing a polymer layer on said masking layer said polymerlayer being capable of acting as a scaffold for the transfer of saidgraphitic substrate to another support;

(IV) transferring the graphitic substrate from said metal catalyst layerto a support;

(V) optionally removing the polymer layer, and optionally depositing afurther oxide or nitride masking layer on top of the upper layerpresent;

(VI) introducing a plurality holes through all layers presentpenetrating to said substrate;

(VII) growing a plurality of semiconducting group III-V nanowires ornanopyramids in the holes, preferably via a molecular beam epitaxy ormetalorganic vapour phase epitaxy.

The polymer layer is one that can be used as an e-beam resist and arewell known in the art. Suitable polymers include poly(meth)acrylates,copolymer resists composed of copolymers based on methyl methacrylateand methacrylic acid (PMMA/MA), styrene acrylates, Novolak based e-beamresists, epoxy based polymer resins, other acrylate polymers,glutarimide, phenol formaldehyde polymers and etc.

The polymer layer may be 100˜2000 nm in thickness.

Patterning

The nanowires or nanopyramids need to grow from the graphitic substrate.That means that holes need to be patterned through all upper layerspresent such as the seed layer and masking layer(s) if present, to thesubstrate. Etching of these holes is a well-known process and can becarried out using e-beam lithography or any other known techniques. Thehole patterns in the mask can be easily fabricated using conventionalphoto/e-beam lithography or nanoimprinting. Focussed ion beam technologymay also be used in order to create a regular array of nucleation siteson the graphitic surface for the nanowire or nanopyramid growth. Theholes created in the masking and seed layers can be arranged in anypattern which is desired.

Holes are preferably substantially circular in cross section. The depthof each hole will be the same as the thickness of the seed layers andmasking layers. The diameter of the holes is preferably up to 500 nm,such as up to 100 nm, ideally up to 20 to 200 nm. The diameter of thehole sets a maximum diameter for the size of the nanowires ornanopyramids so the hole size and nanowire or nanopyramid diametersshould match. However, nanowire or nanopyramid diameter larger than thehole size could be achieved by adopting a core-shell nanowire ornanopyramid geometry.

The number of holes is a function of the area of the substrate anddesired nanowire or nanopyramid density.

As the nanowires or nanopyramids begin growing within a hole, this tendsto ensure that the initial growth of the nanowires or nanopyramids issubstantially perpendicular to the substrate. This is a furtherpreferred feature of the invention. One nanowire or nanopyramidpreferably grows per hole.

Growth of Nanowires or Nanopyramids

In order to prepare nanowires or nanopyramids of commercial importance,it is preferred that these grow epitaxially on the substrate. It is alsoideal if growth occurs perpendicular to the substrate and ideallytherefore in the [111] (for cubic crystal structure) or [0001] (forhexagonal crystal structure) direction.

The present inventors have determined, however, that epitaxial growth ongraphitic substrates is possible by determining a possible lattice matchbetween the atoms in the semiconductor nanowire or nanopyramid and thecarbon atoms in the graphene sheet.

The carbon-carbon bond length in graphene layers is about 0.142 nm.Graphite has hexagonal crystal geometry. The present inventors havepreviously realised that graphite can provide a substrate on whichsemiconductor nanowires or nanopyramids can be grown as the latticemismatch between the growing nanowire or nanopyramid material and thegraphitic substrate can be very low.

The inventors have realised that due to the hexagonal symmetry of thegraphitic substrate and the hexagonal symmetry of the semiconductoratoms in the (111) planes of a nanowire or nanopyramid growing in the[111] direction with a cubic crystal structure (or in the (0001) planesof a nanowire or nanopyramid growing in the [0001] direction with ahexagonal crystal structure), a lattice match can be achieved betweenthe growing nanowires or nanopyramids and the substrate. A comprehensiveexplanation of the science here can be found in WO2013/104723.

Without wishing to be limited by theory, due to the hexagonal symmetryof the carbon atoms in graphitic layers, and the hexagonal symmetry ofthe atoms of cubic or hexagonal semiconductors in the [111] and [0001]crystal direction, respectively, (a preferred direction for mostnanowire or nanopyramid growth), a close lattice match between thegraphitic substrate and semiconductor can be achieved when thesemiconductor atoms are placed above the carbon atoms of the graphiticsubstrate, ideally in a hexagonal pattern. This is a new and surprisingfinding and can enable the epitaxial growth of nanowires or nanopyramidson graphitic substrates.

The different hexagonal arrangements of the semiconductor atoms asdescribed in WO2013/104723, can enable semiconductor nanowires ornanopyramids of such materials to be vertically grown to formfree-standing nanowires or nanopyramids on top of a thin carbon-basedgraphitic material.

In a growing nanopyramid, the triangular faces are normally terminatedwith (1-101) or (1-102) planes. The triangular side surfaces with(1-101) facets could either converge to a single point at the tip orcould form a new facets ((1-102)planes) before converging at the tip. Insome cases, the nanopyramids are truncated with its top terminated with{0001} planes.

Whilst it is ideal that there is no lattice mismatch between a growingnanowire or nanopyramid and the substrate, nanowires or nanopyramids canaccommodate much more lattice mismatch than thin films for example. Thenanowires or nanopyramids of the invention may have a lattice mismatchof up to about 10% with the substrate and epitaxial growth is stillpossible. Ideally, lattice mismatches should be 7.5% or less, e.g. 5% orless.

For some semiconductors like cubic InAs (a=6.058 Å), cubic GaSb (a=6.093Å), the lattice mismatch is so small (<˜1%) that excellent growth ofthese semiconductor nanowires or nanopyramids can be expected.

Growth of nanowires/nanopyramids can be controlled through flux ratios.Nanopyramids are encouraged, for example if high group V flux isemployed.

The nanowires that are grown can be said to be in essentially inone-dimensional form with nanometer dimensions in their width ordiameter and their length typically in the range of a few 100 nm to afew μm. Ideally the nanowire diameter is not greater than 500 nm.Ideally the nanowire diameter is between 50 and 500 nm; however, thediameter can exceed few micrometers (called microwires).

The nanowire grown in the present invention may therefore be from 250 nmto several micrometers in length, e.g. up to 5 micrometers. Preferablythe nanowires are at least 1 micrometer in length. Where a plurality ofnanowires are grown, it is preferred if they all meet these dimensionrequirements. Ideally, at least 90% of the nanowires grown on asubstrate will be at least 1 micrometer in length. Preferablysubstantially all the nanowires will be at least 1 micrometer in length.

Nanopyramids may be 250 nm to 1 micrometer in height, such as 400 to 800nm in height, such as about 500 nm.

Moreover, it will be preferred if the nanowires or nanopyramids grownhave the same dimensions, e.g. to within 10% of each other. Thus, atleast 90% (preferably substantially all) of the nanowires ornanopyramids on a substrate will preferably be of the same diameterand/or the same length (i.e. to within 10% of the diameter/length ofeach other). Essentially, therefore the skilled man is looking forhomogeneity and nanowires or nanopyramids that are substantially thesame in terms of dimensions.

The length of the nanowires or nanopyramids is often controlled by thelength of time for which the growing process runs. A longer processtypically leads to a (much) longer nanowire.

The nanowires or nanopyramids have typically a hexagonal cross sectionalshape. The nanowire may have a cross sectional diameter of 25 nm toseveral micrometers (i.e. its thickness). As noted above, the diameteris ideally constant throughout the majority of the nanowire. Nanowirediameter can be controlled by the manipulation of the substratetemperature and/or the ratio of the atoms used to make the nanowire asdescribed further below.

Moreover, the length and diameter of the nanowires or nanopyramids canbe affected by the temperature at which they are formed. Highertemperatures encourage high aspect ratios (i.e. longer and/or thinnernanowires). The skilled man is able to manipulate the growing process todesign nanowires or nanopyramids of desired dimensions.

The nanowires or nanopyramids of the invention are formed from at leastone III-V compound. Group III options are B, Al, Ga, In, and TI.Preferred options here are Ga, Al and In.

Group V options are N, P, As, Sb. All are preferred.

It is of course possible to use more than one element from group IIIand/or more than one element from group V. Preferred compounds fornanowire or nanopyramid manufacture include AlAs, GaSb, GaP, GaN, AlN,AlGaN, AlGaInN, GaAs, InP, InN, InGaAs, InSb, InAs, or AlGaAs. Compoundsbased on Al, Ga and In in combination with N are one option. The use ofGaN, AlGaN, AlInGaN or AlN is highly preferred.

It is most preferred if the nanowires or nanopyramids consist of Ga, Al,In and N (along with any doping atoms as discussed below).

Whilst the use of binary materials is possible, the use of ternarynanowires or nanopyramids in which there are two group III cations witha group V anion are preferred here, such as AlGaN. The ternary compoundsmay therefore be of formula XYZ wherein X is a group III element, Y is agroup III different from X, and Z is a group V element. The X to Y molarratio in XYZ is preferably 0.1 to 0.9, i.e. the formula is preferablyX_(x)Y_(1-x)Z where subscript x is 0.1 to 0.9.

Quaternary systems might also be used and may be represented by theformula A_(x)B_(1-x)C_(y)D_(1-y) where A and B are group III elementsand C and D are group V elements. Again subscripts x and y are typically0.1 to 0.9. Other options will be clear to the skilled man.

Doping

The nanowires or nanopyramids of the invention can contain a p-n orp-i-n junction, e.g. to enable their use in LEDs. NWs or nanopyramids ofthe invention are therefore optionally provided with an undopedintrinsic semiconductor region between a p-type semiconductor and ann-type semiconductor region.

It is therefore essential that the nanowires or nanopyramids are doped.Doping typically involves the introduction of impurity ions into thenanowire, e.g. during MBE or MOVPE growth. The doping level can becontrolled from ˜10¹⁵/cm³ to 10²⁰/cm³. The nanowires or nanopyramids canbe p-type doped or n-type doped as desired. Doped semiconductors areextrinsic semiconductors.

The n(p)-type semiconductors have a larger electron (hole) concentrationthan hole (electron) concentration by doping an intrinsic semiconductorwith donor (acceptor) impurities. Suitable donor (acceptors) for III-Vcompounds can be Tc, Sn (Be, Mg and Zn). Si can be amphoteric, eitherdonor or acceptor depending on the site where Si goes to, depending onthe orientation of the growing surface and the growth conditions.Dopants can be introduced during the growth process or by ionimplantation of the nanowires or nanopyramids after their formation.

Higher carrier injection efficiency is required to obtain higherexternal quantum efficiency (EQE) of LEDs. However, the increasingionization energy of Mg acceptors with increasing Al content in AlGaNalloys makes it difficult to obtain higher hole concentration in AlGaNalloys with higher Al content. To obtain higher hole injectionefficiency (especially in the cladding/barrier layers consisting of highAl content), the inventors have devised a number of strategies which canbe used individually or together.

There are problems to overcome in the doping process therefore. It ispreferred if the nanowires or nanopyramids of the invention comprise Al.The use of Al is advantageous as high Al content leads to high bandgaps, enabling UV-C LED emission from the active layer(s) of nanowiresor nanopyramids and/or avoiding absorption of the emitted light in thedoped cladding/barrier layers. Where the band gap is high, it is lesslikely that UV light is absorbed by this part of the nanowires ornanopyramids. The use therefore of AlN or AlGaN in nanowires ornanopyramids is preferred.

However, p-type doping of AlGaN or AlN to achieve high electricalconductivity (high hole concentration) is challenging as the ionizationenergy of Mg or Be acceptors increases with increasing Al content inAlGaN alloys. The present inventors propose various solutions tomaximise electrical conductivity (i.e. maximise hole concentration) inAlGaN alloys with higher average Al content.

Where the nanowires or nanopyramids comprise AlN or AlGaN, achievinghigh electrical conductivity by introducing p-type dopants is achallenge. One solution relies on a short period superlattice (SPSL). Inthis method, we grow a superlattice structure consisting of alternatinglayers with different Al content instead of a homogeneous AlGaN layerwith higher Al composition. For example, the cladding layer with 35% Alcontent could be replaced with a 1.8 to 2.0 nm thick SPSL consisting of,for example, alternating Al_(x)Ga_(1-x)N:Mg/Al_(y)Ga_(1-y)N:Mg withx=0.30/y=0.40. The low ionization energy of acceptors in layers withlower Al composition leads to improved hole injection efficiency withoutcompromising on the barrier height in the cladding layer. This effect isadditionally enhanced by the polarization fields at the interfaces. TheSPSL can be followed with a highly p-doped GaN:Mg layer for better holeinjection.

More generally, the inventors propose to introduce a p-type dopedAl_(x)Ga_(1-x)N/Al_(y)Ga_(1-y)N short period superlattice (i.e.alternating thin layers of Al_(x)Ga_(1-x)N and Al_(y)Ga_(1-y)N) into thenanowires or nanopyramid structure, where the Al mole fraction x is lessthan y, instead of a p-type doped Al_(z)Ga_(1-z)N alloy where x<z<y. Itis appreciated that x could be as low as 0 (i.e. GaN) and y could be ashigh as 1 (i.e. AlN). The superlattice period should preferably be 5 nmor less, such as 2 nm, in which case the superlattice will act as asingle Al_(z)Ga_(1-z)N alloy (with z being a layer thickness weightedaverage of x and y) but with a higher electrical conductivity than thatof the Al_(z)Ga_(1-z)N alloy, due to the higher p-type doping efficiencyfor the lower Al content Al_(x)Ga_(1-x)N layers.

In the nanowires or nanopyramids comprising a p-type doped superlattice,it is preferred if the p-type dopant is an alkali earth metal such as Mgor Be.

A further option to solve the problem of doping an Al containingnanowire/nanopyramid follows similar principles. Instead of asuperlattice containing thin AlGaN layers with low or no Al content, ananostructure can be designed containing a gradient of Al content (molefraction) in the growth direction of the AlGaN within the nanowires ornanopyramids. Thus, as the nanowires or nanopyramids grow, the Alcontent is reduced/increased and then increased/reduced again to createan Al content gradient within the nanowires or nanopyramids.

This may be called polarization doping. In one method, the layers aregraded either from GaN to AlN or AlN to GaN. The graded region from GaNto AlN and AlN to GaN may lead to n-type and p-type conduction,respectively. This can happen due to the presence of dipoles withdifferent magnitude compared to its neighbouring dipoles. The GaN to AlNand AlN to GaN graded regions can be additionally doped with n-typedopant and p-type dopant respectively.

In a preferred embodiment, p-type doping is used in AlGaN nanowiresusing Be as a dopant.

Thus, one option would be to start with a GaN nanowire/nanopyramid andincrease Al and decrease Ga content gradually to form AlN, perhaps overa growth thickness of 100 nm. This graded region could act as a p- orn-type region, depending on the crystal plane, polarity and whether theAl content is decreasing or increasing in the graded region,respectively. Then the opposite process is effected to produce GaN oncemore to create an n- or p-type region (opposite to that previouslyprepared). These graded regions could be additionally doped with n-typedopants such as Si and p-type dopants such as Mg or Be to obtain n- orp-type regions with high charge carrier density, respectively. Thecrystal planes and polarity is governed by the type ofnanowire/nanopyramid as is known in the art.

Viewed from another aspect therefore, the nanowires or nanopyramids ofthe invention comprise Al, Ga and N atoms wherein during the growth ofthe nanowires or nanopyramids the concentration of Al is varied tocreate an Al concentration gradient within the nanowires ornanopyramids.

In a third embodiment, the problem of doping in an Al containingnanowire or nanopyramid is addressed using a tunnel junction. A tunneljunction is a barrier, such as a thin layer, between two electricallyconducting materials. In the context of the present invention, thebarrier functions as an ohmic electrical contact in the middle of asemiconductor device.

In one method, a thin electron blocking layer is inserted immediatelyafter the active region, which is followed by a p-type doped AlGaNcladding layer with Al content higher than the Al content used in theactive layers. The p-type doped cladding layer is followed by a highlyp-type doped cladding layer and a very thin tunnel junction layerfollowed by an n-type doped AlGaN layer. The tunnel junction layer ischosen such that the electrons tunnel from the valence band in p-AlGaNto the conduction band in the n-AlGaN, creating holes that are injectedinto the p-AlGaN layer.

More generally, it is preferred if the nanowire or nanopyramid comprisestwo regions of doped GaN (one p- and one n-doped region) separated by anAl layer, such as a very thin Al layer. The Al layer might be a few nmthick such as 1 to 10 nm in thickness. It is appreciated that there areother optional materials that can serve as a tunnel junction whichincludes highly doped InGaN layers.

It is particularly surprising that doped GaN layers can be grown on theAl layer.

In one embodiment therefore, the invention provides a nanowire ornanopyramid having a p-type doped (Al)GaN region and an n-type doped(Al)GaN region separated by an Al layer.

The nanowires or nanopyramids of the invention can be grown to have aheterostructured form radially or axially. For example for an axialheterostructured nanowire or nanopyramid, p-n junction can be axiallyformed by growing a p-type doped core first, and then continue with ann-doped core (or vice versa). For a radially heterostructured nanowireor nanopyramid, p-n junction can be radially formed by growing thep-type doped nanowire or nanopyramid core first, and then the n-typedoped semiconducting shell is grown (or vice versa)—a core shellnanowire. An intrinsic shell can be positioned between doped regions fora p-i-n nanowire. The NWs or nanopyramids are grown axially or radiallyand are therefore formed from a first section and a second section. Thetwo sections are doped differently to generate a p-n junction or p-i-njunction. The first or second section of the NW or nanopyramid is thep-type doped or n-type doped section.

The nanowires or nanopyramids of the invention preferably growepitaxially. They attach to the underlying substrate through covalent,ionic or quasi van der Waals binding. Accordingly, at the junction ofthe substrate and the base of the nanowire, crystal planes are formedepitaxially within the nanowire. These build up, one upon another, inthe same crystallographic direction thus allowing the epitaxial growthof the nanowire. Preferably the nanowires or nanopyramids growvertically. The term vertically here is used to imply that the nanowiresor nanopyramids grow perpendicular to the support. It will beappreciated that in experimental science the growth angle may not beexactly 90° but the term vertically implies that the nanowires ornanopyramids are within about 10° of vertical/perpendicular, e.g. within5°. Because of the epitaxial growth via covalent, ionic or quasi van derWaals bonding, it is expected that there will be an intimate contactbetween the nanowires or nanopyramids and the substrate. To enhance theelectrical contact property further, the substrate, can be doped tomatch the major carriers of grown nanowires or nanopyramids.

Because nanowires or nanopyramids are epitaxially grown involvingphysical and chemical bonding to substrates at high temperature, thebottom contact is preferably ohmic.

It will be appreciated that the substrate comprises a plurality ofnanowires or nanopyramids. Preferably the nanowires or nanopyramids growabout parallel to each other. It is preferred therefore if at least 90%,e.g. at least 95%, preferably substantially all nanowires ornanopyramids grow in the same direction from the same plane of thesubstrate.

It will be appreciated that there are many planes within a substratefrom which epitaxial growth could occur. It is preferred ifsubstantially all nanowires or nanopyramids grow from the same plane. Itis preferred if that plane is parallel to the substrate surface. Ideallythe grown nanowires or nanopyramids are substantially parallel.Preferably, the nanowires or nanopyramids grow substantiallyperpendicular to the substrate.

The nanowires of the invention should preferably grow in the [111]direction for nanowires or nanopyramids with cubic crystal structure and[0001] direction for nanowires or nanopyramids with hexagonal crystalstructure. If the crystal structure of the growing nanowire ornanopyramid is cubic, then the (111) interface between the nanowire ornanopyramid and the graphitic substrate represents the plane from whichaxial growth takes place. If the nanowire or nanopyramid has a hexagonalcrystal structure, then the (0001) interface between the nanowire ornanopyramid and the graphitic substrate represents the plane from whichaxial growth takes place. Planes (11) and (0001) both represent the same(hexagonal) plane of the nanowire, it is just that the nomenclature ofthe plane varies depending on the crystal structure of the growingnanowire.

The nanowires or nanopyramids are preferably grown by MBE or MOVPE. Inthe MBE method, the substrate is provided with a molecular beam of eachreactant, e.g. a group III element and a group V element preferablysupplied simultaneously. A higher degree of control of the nucleationand growth of the nanowires or nanopyramids on the graphitic substratemight be achieved with the MBE technique by using migration-enhancedepitaxy (MEE) or atomic-layer MBE (ALMBE) where e.g. the group III and Velements can be supplied alternatively.

A preferred technique is solid-source MBE, in which very pure elementssuch as gallium and arsenic are heated in separate effusion cells, untilthey begin to slowly evaporate (e.g. gallium) or sublimate (e.g.arsenic). The gaseous elements then condense on the substrate, wherethey may react with each other. In the example of gallium and arsenic,single-crystal GaAs is formed. The use of the term “beam” implies thatevaporated atoms (e.g. gallium) or molecules (e.g. As₄ or As₂) do notinteract with each other or vacuum chamber gases until they reach thesubstrate.

MBE takes place in ultra-high vacuum, with a background pressure oftypically around 10⁻¹⁰ to 10⁻⁹ Torr. Nanostructures are typically grownslowly, such as at a speed of up to a few, such as about 10, μm perhour. This allows nanowires or nanopyramids to grow epitaxially andmaximises structural performance.

In the MOVPE method, the substrate is kept in a reactor in which thesubstrate is provided with a carrier gas and a metal organic gas of eachreactant, e.g. a metal organic precursor containing a group III elementand a metal organic precursor containing a group V element preferablysupplied simultaneously. The typical carrier gases are hydrogen,nitrogen or a mixture of the two. A higher degree of control of thenucleation and growth of the nanowires or nanopyramids on the graphiticsubstrate might be achieved with the MOVPE technique by using pulsedlayer growth technique, where e.g. the group III and V elements can besupplied alternatively.

Selective Area Growth of Nanowires or Nanopyramids

The nanowires or nanopyramids of the invention may be grown by selectivearea growth (SAG) method, e.g. in the case of III-nitride nanowire.Inside the growth chamber, the graphitic substrate temperature can thenbe set to a temperature suitable for the growth of the nanowire ornanopyramid in question. The growth temperature may be in the range 300to 1000° C. The temperature employed is, however, specific to the natureof the material in the nanowire. For GaN, a preferred temperature is 700to 950° C., e.g. 800 to 900° C., such as 810° C. For AlGaN the range isslightly higher, for example 800 to 980° C., such as 830 to 950° C.,e.g. 850° C.

It will be appreciated therefore that the nanowires or nanopyramids cancomprise different group III-V semiconductors within the nanowire, e.g.starting with a GaN stem followed by an AlGaN component or AlGaInNcomponent and so on.

Nanowire growth can be initiated by opening the shutter of the Gaeffusion cell, the nitrogen plasma cell, and the dopant cellsimultaneously initiating the growth of doped GaN nanowires ornanopyramids, hereby called as stem. The length of the GaN stem can bekept between 10 nm to several 100 s of nanometers. Subsequently, onecould increase the substrate temperature if needed, and open the Alshutter to initiate the growth of AlGaN nanowires or nanopyramids. Onecould initiate the growth of AlGaN nanowires or nanopyramids ongraphitic layers without the growth of GaN stem. n- and p-dopednanowires or nanopyramids can be obtained by opening the shutter of then-dopant cell and p-dopant cell, respectively, during the nanowire ornanopyramid growth. For ex: Si dopant cell for n-doping of nanowires ornanopyramids, and Mg dopant cell for p-doping of nanowires ornanopyramids.

The temperature of the effusion cells can be used to control growthrate. Convenient growth rates, as measured during conventional planar(layer by layer) growth, are 0.05 to 2 μm per hour, e.g. 0.1 μm perhour. The ratio of Al/Ga can be varied by changing the temperature ofthe effusion cells.

The pressure of the molecular beams can also be adjusted depending onthe nature of the nanowire or nanopyramid being grown. Suitable levelsfor beam equivalent pressures are between 1×10⁻⁷ and 1×10⁻⁴ Torr.

The beam flux ratio between reactants (e.g. group III atoms and group Vmolecules) can be varied, the preferred flux ratio being dependent onother growth parameters and on the nature of the nanowire or nanopyramidbeing grown. In the case of nitrides, nanowires or nanopyramids arealways grown under nitrogen rich conditions.

It is an embodiment of the invention to employ a multistep, such as twostep, growth procedure, e.g. to separately optimize the nanowire ornanopyramid nucleation and nanowire or nanopyramid growth.

A significant benefit of MOVPE is that the nanowires or nanopyramids canbe grown at a much faster growth rate. This method favours the growth ofradial heterostructure nanowires or nanopyramids and microwires, forexample: n-doped GaN core with shell consisting of intrinsicAlN/Al(In)GaN multiple quantum wells (MQW), AlGaN electron blockinglayer (EBL), and p-doped (Al)GaN shell. This method also allows thegrowth of axial heterostructured nanowire or nanopyramid usingtechniques such as pulsed growth technique or continuous growth modewith modified growth parameters for e.g., lower V/III molar ratio andhigher substrate temperature.

In more detail, the reactor must be evacuated after placing the sample,and is purged with N₂ to remove oxygen and water in the reactor. This isto avoid any damage to the graphene at the growth temperatures, and toavoid unwanted reactions of oxygen and water with the precursors. Thetotal pressure is set to be between 50 and 400 Torr. After purging thereactor with N₁, the substrate is thermally cleaned under H₂ atmosphereat a substrate temperature of about 1200° C. The substrate temperaturecan then be set to a temperature suitable for the growth of the nanowireor nanopyramid in question. The growth temperature may be in the range700 to 1200° C. The temperature employed is, however, specific to thenature of the material in the nanowire. For GaN, a preferred temperatureis 800 to 1150° C., e.g. 900 to 1100° C., such as 1100° C. For AlGaN therange is slightly higher, for example 900 to 1250° C., such as 1050 to1250° C., e.g. 1250° C.

The metal organic precursors for the nanowire or nanopyramid growth canbe either trimethylgallium (TMGa), or triethylgallium (TEGa) for Ga,trimethylalumnium (TMAl) or triethylaluminum (TEAl) for Al, andtrimethylindium (TMIn) or triethylindium (TEIn) for In. The precursorsfor dopants can be SiH₄ for silicon and bis(cyclopentadienyl)magnesium(Cp₂Mg) or bis(methylcyclopentadienyl)magnesium ((MeCp)₂Mg) for Mg. Theflow rate of TMGa, TMAl and TMIn can be maintained between 5 and 100seem. The NH₃ flow rate can be varied between 5 and 150 sccm.

In particular, the simple use of vapour-solid growth may enable nanowireor nanopyramid growth. Thus, in the context of MBE, simple applicationof the reactants. e.g. In and N, to the substrate without any catalystcan result in the formation of a nanowire. This forms a further aspectof the invention which therefore provides the direct growth of asemiconductor nanowire or nanopyramid formed from the elements describedabove on a graphitic substrate. The term direct implies therefore theabsence of a film of catalyst to enable growth.

Catalyst-Assisted Growth of Nanowires or Nanopyramids

The nanowires or nanopyramids of the invention may also be grown in thepresence of a catalyst. A catalyst can be introduced into those holes toprovide nucleating sites for nanowire or nanopyramid growth. Thecatalyst can be one of the elements making up the nanowire ornanopyramid so-called self-catalysed, or different from any of theelements making up the nanowire.

For catalyst-assisted growth the catalyst may be Au or Ag or thecatalyst may be a metal from the group used in the nanowire ornanopyramid growth (e.g. group III metal), especially one of the metalelements making up the actual nanowire or nanopyramid (self-catalysis).It is thus possible to use another element from group III as a catalystfor growing a III-V nanowire or nanopyramid e.g. use Ga as a catalystfor a Ga-group V nanowire or nanopyramid and so on. Preferably thecatalyst is Au or the growth is self-catalysed (i.e. Ga for a Ga-group Vnanowire or nanopyramid and so on). The catalyst can be deposited ontothe substrate in the holes patterned through the seed layer/maskinglayer to act as a nucleation site for the growth of the nanowires ornanopyramids. Ideally, this can be achieved by providing a thin film ofcatalytic material formed over the seed layer or masking layer afterholes have been etched in the layers. When the catalyst film is meltedas the temperature increases to the NW or nanopyramid growthtemperature, the catalyst forms nanometre sized particle-like dropletson the substrate and these droplets form the points where nanowires ornanopyramids can grow.

This is called vapour-liquid-solid growth (VLS) as the catalyst is theliquid, the molecular beam is the vapour and the nanowire or nanopyramidprovides the solid component. In some cases the catalyst particle canalso be solid during the nanowire or nanopyramid growth, by a so calledvapour-solid-solid growth (VSS) mechanism. As the nanowire ornanopyramid grows (by the VLS method), the liquid (e.g. gold) dropletstays on the top of the nanowire. It remains at the top of the nanowireor nanopyramid after growth and may therefore play a major role incontacting a top electrode.

As noted above, it is also possible to prepare self-catalysed nanowiresor nanopyramids. By self-catalysed is meant that one of the componentsof the nanowire or nanopyramid acts as a catalyst for its growth.

For example, a Ga layer can be applied to the seed/masking layer, meltedto form droplets acting as nucleation sites for the growth of Gacontaining nanowires or nanopyramids. Again, a Ga metal portion may endup positioned on the top of the nanowire.

In more detail, a Ga/In flux can be supplied to the substrate surfacefor a period of time to initiate the formation of Ga/In droplets on thesurface upon heating of the substrate. The substrate temperature canthen be set to a temperature suitable for the growth of the nanowire ornanopyramid in question. The growth temperature may be in the range 300to 700° C. The temperature employed is, however, specific to the natureof the material in the nanowire, the catalyst material and the substratematerial. For GaAs, a preferred temperature is 540 to 630° C., e.g. 590to 630° C., such as 610° C. For InAs the range is lower, for example 420to 540° C., such as 430 to 540° C., e.g. 450° C.

Nanowire growth can be initiated by opening the shutter of the Ga/Ineffusion cell and the counter ion effusion cell, simultaneously once acatalyst film has been deposited and melted.

The temperature of the effusion cells can be used to control growthrate. Convenient growth rates, as measured during conventional planar(layer by layer) growth, are 0.05 to 2 μm per hour, e.g. 0.1 μm perhour.

The pressure of the molecular beams can also be adjusted depending onthe nature of the nanowire or nanopyramid being grown. Suitable levelsfor beam equivalent pressures are between 1×10⁻⁷ and 1×10⁻⁵ Torr.

The beam flux ratio between reactants (e.g. group III atoms and group Vmolecules) can be varied, the preferred flux ratio being dependent onother growth parameters and on the nature of the nanowire or nanopyramidbeing grown.

It has been found that the beam flux ratio between reactants can affectcrystal structure of the nanowire. For example, using Au as a catalyst,growth of GaAs nanowires or nanopyramids with a growth temperature of540° C., a Ga flux equivalent to a planar (layer by layer) growth rateof 0.6 μm per hour, and a beam equivalent pressure (BEP) of 9×10⁻⁶ Torrfor As₄ produces wurtzite crystal structure. As opposed to this, growthof GaAs nanowires or nanopyramids at the same growth temperature, butwith a Ga flux equivalent to a planar growth rate of 0.9 μm per hour anda BEP of 4×10⁻⁶ Torr for As₄, produces zinc blende crystal structure.

Nanowire diameter can in some cases be varied by changing the growthparameters. For example, when growing self-catalyzed GaAs nanowires ornanopyramids under conditions where the axial nanowire or nanopyramidgrowth rate is determined by the As₄ flux, the nanowire or nanopyramiddiameter can be increased/decreased by increasing/decreasing the Ga:As₄flux ratio. The skilled man is therefore able to manipulate the nanowireor nanopyramid in a number of ways. Moreover, the diameter could also bevaried by growing a shell around the nanowire or nanopyramid core,making a core-shell geometry.

It is thus an embodiment of the invention to employ a multistep, such astwo step, growth procedure, e.g. to separately optimize the nanowire ornanopyramid nucleation and nanowire or nanopyramid growth.

Moreover, the size of the holes can be controlled to ensure that onlyone nanowire or nanopyramid can grow in each hole. It is thereforepreferred if only one nanowire or nanopyramid grows per hole in themask. Finally, the holes can be made of a size where the droplet ofcatalyst that forms within the hole is sufficiently large to allownanowire or nanopyramid growth. In this way, a regular array ofnanowires or nanopyramids can be grown, even using Au catalysis.

Top Contact

In order to create some devices of the invention, the top of thenanowires or nanopyramids needs to comprise a top contact.

In one preferred embodiment, a top contact is formed using anothergraphitic layer. The invention then involves placing a graphitic layeron top of the formed nanowires or nanopyramids to make a top contact. Itis preferred that the graphitic top contact layer is substantiallyparallel with the substrate layer. It will also be appreciated that thearea of the graphitic layer does not need to be the same as the area ofthe substrate. It may be that a number of graphitic layers are requiredto form a top contact with a substrate with an array of nanowires ornanopyramids.

The graphitic layers used can be the same as those described in detailabove in connection with the substrate. The top contact is graphitic,more especially it is graphene. This graphene substrate should containno more than 10 layers of graphene or its derivatives, preferably nomore than 5 layers (which is called as a few-layered graphene).Especially preferably, it is a one-atom-thick planar sheet of graphene.

The crystalline or “flake” form of graphite consists of many graphenesheets stacked together (i.e. more than 10 sheets). It is preferred ifthe top contact is 20 nm in thickness or less. Even more preferably, thegraphitic top contact may be 5 nm or less in thickness.

When graphene contacts directly to the semiconductor nanowires ornanopyramids, it usually forms a Schottky contact which hinders theelectrical current flow by creating a barrier at the contact junction.Due to this problem, the research on graphene deposited onsemiconductors has been mainly confined to the use ofgraphene/semiconductor Schottky junctions.

Application of the top contact to the formed nanowires or nanopyramidscan be achieved by any convenient method. Methods akin to thosementioned previously for transferring graphitic layers to substratecarriers may be used. The graphitic layers from Kish graphite, highlyordered pyrolytic graphite (HOPG), or CVD may be exfoliated bymechanical or chemical methods. Then they can be transferred intoetching solutions such as HF or acid solutions to remove Cu (Ni, Pt,etc.) (especially for CVD grown graphitic layers) and any contaminantsfrom the exfoliation process. The etching solution can be furtherexchanged into other solutions such as deionised water to clean thegraphitic layers. The graphitic layers can then be easily transferredonto the formed nanowires or nanopyramids as the top contact. Againe-beam resist or photoresist may be used to support the thin graphiticlayers during the exfoliation and transfer processes, which can beremoved easily after deposition.

It is preferred if the graphitic layers are dried completely afteretching and rinsing, before they are transferred to the top of thenanowire or nanopyramid arrays. To enhance the contact between graphiticlayers and nanowires or nanopyramids a mild pressure and heat can beapplied during this “dry” transfer.

Alternatively, the graphitic layers can be transferred on top of thenanowire or nanopyramid arrays, together with a solution (e.g. deionisedwater). As the solution dries off, the graphitic layers naturally form aclose contact to underlying nanowires or nanopyramids. In this “wet”transfer method, the surface tension of the solution during the dryingprocess might bend or knock out the nanowire or nanopyramid arrays. Toprevent this, where this wet method is used, more robust nanowires ornanopyramids are preferably employed. Nanowires having a diameter of >80nm might be suitable. One may also use the critical-point dryingtechnique to avoid any damage caused by surface tension during thedrying process. Another way to prevent this is to use supporting andelectrically isolating material as fill-in material between nanowires ornanopyramids.

If there is a water droplet on a nanowire or nanopyramid array andattempts to remove it involve, for example a nitrogen blow, the waterdrop will become smaller by evaporation, but the drop will always try tokeep a spherical form due to surface tension. This could damage ordisrupt the nanostructures around or inside the water droplet.

Critical point drying circumvents this problem. By increasingtemperature and pressure, the phase boundary between liquid and gas canbe removed and the water can be removed easily.

Also doping of the graphitic top contact can be utilized. The majorcarrier of the graphitic top contact can be controlled as either holesor electrons by doping. It is preferable to have the same doping type inthe graphitic top contact and in the semiconducting nanowires ornanopyramids.

It will be appreciated therefore that both top graphitic layer and thesubstrate can be doped. In some embodiments, the substrate and/or thegraphitic layer is doped by a chemical method which involves with anadsorption of organic or inorganic molecules such as metal chlorides(FeCl₃, AuCl₃ or GaCl₃), NO₂, HNO₃, aromatic molecules or chemicalsolutions such as ammonia.

The surface of substrate and/or the graphitic layer could also be dopedby a substitutional doping method during its growth with incorporationof dopants such as B, N, S, or Si.

Applications

Semiconductor nanowires or nanopyramids have wide ranging utility. Theyare semiconductors so can be expected to offer applications in any fieldwhere semiconductor technology is useful. They are primarily of use inintegrated nanoelectronics and nano-optoelectronic applications.

An ideal device for their deployment might be a solar cell, LED orphotodetector. One possible device is a nanowire or nanopyramid solarcell sandwiched between two graphene layers as the two terminals.

Such a solar cell has the potential to be efficient, cheap and flexibleat the same time. This is a rapidly developing field and furtherapplications on these valuable materials will be found in the nextyears. The same concept can be used to also fabricate otheropto-electronic devices such as light-emitting diodes (LEDs), waveguidesand lasers.

It will be appreciated that devices of the invention are provided withelectrodes to enable charge to be passed into the device.

The invention will now be further discussed in relation to the followingnon limiting examples and figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1a shows a flow diagram for the manufacture of a composition asclaimed herein. Graphene layer 1 is carried on a support 2. A thin seedlayer 3 is evaporated onto the graphene layer. In the first embodiment,the seed layer oxidises to form oxidised seed layer 4 and holes 5 arepatterned through the oxidised seed layer to the graphitic substratebelow. Nanowires are then grown in the holes.

In alternative process, a metal oxide or nitride layer 6 is deposited onthe seed layer 3, before holes 5 are patterned through both seed andmasking layer. Nanowires are then grown in the holes.

FIG. 1b shows a flow diagram in which a graphene substrate 1 is grown ona metal catalyst layer 10 in the form of a foil or film. A seed layer 3is then applied to the graphene. This seed layer can be oxidised tolayer 4 and/or also a masking layer 6 can be applied. The graphene layer1 is then removed from metal catalyst layer 10 and transferred to a newsubstrate 11 by an electrochemical delamination method where an aqueouselectrolyte based on acid, hydroxide, carbonate, or chloride solution isemployed. There is then the opportunity to deposit a further maskinglayer 6′ before patterning.

FIG. 2a shows a schematic of a graphitic substrate where hole patternsthrough an Al seed layer with two masking layers formed from Al oxideand silica. FIG. 2 b˜l shows core-shell type GaAs nanowires grown on theresulting substrate using MBE. FIG. 2b ˜2 shows GaN nanorods grown onthe resulting substrate using MBE. FIG. 2 b˜c shows GaN micro-pyramidsgrown on the resulting substrate using MOCVD.

FIG. 3a shows a schematic of a graphitic substrate with a Si seed layerwhich is oxidised to silica and a silica mask deposited thereon. FIG. 3bshows GaAs nanowires grown on the resulting substrate using MBE.

FIG. 4: (a) Low magnification and (b) High magnification tilted view SEMimages of GaN nanopyramids grown on patterned single or double layergraphene by MOVPE.

1. A device comprising a composition of matter, the composition ofmatter comprising: a graphitic substrate; a seed layer having athickness of 50 nm or less deposited directly on top of the graphiticsubstrate; and an oxide or nitride masking layer directly on top of theseed layer; wherein a plurality of holes are present through the seedlayer and through the masking layer to the graphitic substrate; andwherein a plurality of nanowires or nanopyramids are grown from thegraphitic substrate in the plurality of holes, wherein the plurality ofnanowires or nanopyramids comprise at least one semiconducting groupIII-V compound.
 2. The device of claim 1, wherein the seed layer is ametallic layer or a layer of oxidized or nitridized metal.
 3. The deviceof claim 1, wherein the graphitic substrate is graphene.
 4. The deviceof claim 1, wherein the nanowires or nanopyramids grow epitaxially fromthe graphitic substrate.
 5. The device of claim 1, wherein the graphiticsubstrate has a thickness of 20 nm or less.
 6. The device of claim 1,wherein the seed layer is a metal layer comprising Sc, Ti, V, Cr, Mn,Fe, Co, Ni, Cu, Zn, B, Al, Si, Ge, Sb, Ta, W, or Nb, or an oxidizedlayer thereof.
 7. The device of claim 1, wherein the masking layercomprises a metal oxide or metal nitride.
 8. The device of claim 1,wherein the masking layer comprises Al₂O₃, TiO₂, SiO₂, AlN, BN, orSi₃N₄.
 9. The device of claim 1, wherein the composition furthercomprises a support, such that the graphitic substrate is carried on thesupport and the seed layer is deposited on top of the graphiticsubstrate opposite the support, and the support comprises asemiconductor substrate, transparent glass, AlN, or silicon carbide. 10.The device of claim 9, wherein the support comprises a semiconductorsubstrate comprising crystalline Si or GaAs with a crystal orientationof [111], [110], or [100] perpendicular to the surface; or a transparentglass of fused silica or fused alumina.
 11. The device of claim 1,wherein the plurality of nanowires or nanopyramids are doped.
 12. Thedevice of claim 1, wherein the plurality of nanowires or nanopyramidsare a plurality of core-shell nanowires or nanopyramids or a pluralityof radially heterostructured nanowires or nanopyramids.
 13. The deviceof claim 1, wherein the plurality of nanowires or nanopyramids are aplurality of axially heterostructured nanowires or nanopyramids.
 14. Thedevice composition of claim 1, further comprising a graphitic topcontact layer on top of the plurality of nanowires or nanopyramids. 15.The device composition of claim 1, wherein the masking layer comprisesat least two different layers.
 16. (canceled)
 17. (canceled) 18.(canceled)
 19. (canceled)
 20. (canceled)
 21. (canceled)
 22. (canceled)23. The device of claim 1, wherein the device comprises a solar cell orLED.
 24. The device in of claim 1, wherein the surface of the graphiticsubstrate is chemically and/or physically modified in the said pluralityof holes to enhance the epitaxial growth of the plurality of nanowiresor nanopyramids.
 25. The device of claim 1, wherein the device comprisesan electronic device.
 26. The device of claim 15, wherein the maskinglayer comprises an oxide layer and a nitride layer.